Servo-controlled gear changes, which are structurally similar to a manual gear change of the traditional type except for the fact that the clutch pedal and the gear selection lever actuated by the driver are replaced by corresponding electrical or hydraulic servo-controls, are becoming increasingly widespread. When using a servo-controlled gear change, the driver simply has to supply the instruction to change to a higher gear or to a lower gear to a transmission control unit and the transmission control unit independently changes gear by acting both on the engine and on the servo-controls associated with the gear change.
When hydraulic servo-controls are used, the drive device of the gear change comprises a hydraulic circuit provided with a tank for the oil forming the control fluid, a pump which pressurises the fluid, a series of electrovalves which receive the pressurised oil from the pump and discharge the oil to the tank, and a series of hydraulic actuators actuated by the electrovalves.
Once the drive device of the gear change has been completed and before the drive device is coupled to the gear change, the hydraulic circuit of the drive device is filled with the oil forming the control fluid. The filling of the hydraulic circuit involves supplying the oil to the tank until it reaches the predetermined level and then actuating the pump and the electrovalves to supply the oil throughout the hydraulic circuit.
A quantity of air, which is either in suspension in the oil or emulsified with the oil, is introduced into the hydraulic circuit when it is being filled. The presence of air in the hydraulic circuit modifies the behaviour of the electrovalves and in particular of the hydraulic actuators. When the hydraulic circuit contains an excessive amount of air, the drive device of the gear change is not able to guarantee nominal performance and does not therefore manage to carry out the gear changes correctly. Consequently, once the filling of the hydraulic circuit is complete, it is necessary to bleed the hydraulic circuit, i.e. to eliminate the surplus air from the hydraulic circuit.
At present, the bleeding of the hydraulic circuit takes place by actuating the drive device on the test bench for a very long period (up to 45 minutes). However, the bleeding method, as well as being very long, is not always efficient as it does not always make it possible to eliminate the air in the hydraulic circuit. When using the above-described method of bleeding, a high percentage of drive devices are therefore returned by customers because there is air in the hydraulic circuit. In particular, the above-described method of bleeding does not always make it possible to eliminate the air in the drive circuit as air bubbles may remain trapped in the interstices of the chambers and a substantial quantity of air may in particular remain emulsified with the oil.
WO9002083 discloses a method and an apparatus for filling a hydraulic brake system using nitrogen or dry air as a desiccant to assure freedom from moisture in brake fluid supplied from a supply tank as well as at the filling location. A main fluid tank has a vacuum over the fluid to deaerate the fluid and has a pump submerged in the fluid to deliver fluid to the brake system; the pump is driven by a submerged motor which is driven by pressurized brake fluid. The brake system is evacuated, the low pressure is monitored for a test interval for leak detection, the system is filled by the pump and excess fluid is returned to the main tank and is replaced by the nitrogen or dry air.
U.S. Pat. No. 3,726,063 discloses a system for removing contaminants such as dissolved and entrained gas, water and solids from fluids; contaminated fluid is atomized and filmed in a very low pressure vacuum to remove gas and water and filters are provided for removal of solids.